A. P. Zhang

Fabrication and performance of GaN electronic devices

Abstract GaN and related materials (especially AlGaN) have recently attracted a lot of interest for applications in high power electronics capable of operation at elevated temperatures. Although the growth and processing technology for SiC, the other viable wide bandgap semiconductor material, is more mature, the AlGaInN system offers numerous advantages. These include wider bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy) for producing very thick layers or even quasi-substrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices, including heterostructure field effect transistors (HFETs), Schottky and p–i–n rectifiers, heterojunction bipolar transistors (HBTs), bipolar junction transistors (BJTs) and metal-oxide semiconductor field effect transistors (MOSFETs). This review focuses on the development of fabrication processes for these devices and the current state-of-the-art in device performance, for all of these structures. We also detail areas where more work is needed, such as reducing defect densities and purity of epitaxial layers, the need for substrates and improved oxides and insulators, improved p-type doping and contacts and an understanding of the basic growth mechanisms.

Electrical effects of plasma damage in p-GaN

The reverse breakdown voltage of p-GaN Schottky diodes was used to measure the electrical effects of high density Ar or H2 plasma exposure. The near surface of the p-GaN became more compensated through introduction of shallow donor states whose concentration depended on ion flux, ion energy, and ion mass. At high fluxes or energies, the donor concentration exceeded 1019 cm−3 and produced p-to-n surface conversion. The damage depth was established as ∼400 A based on electrical and wet etch rate measurements. Rapid thermal annealing at 900 °C under a N2 ambient restored the initial electrical properties of the p-GaN.

Depth and thermal stability of dry etch damage in GaN Schottky diodes

GaN Schottky diodes were exposed to N2 or H2 inductively coupled plasmas prior to deposition of the rectifying contact. Subsequent annealing, wet photochemical etching, or (NH4)2S surface passivation treatments were examined for their effect on diode current–voltage (I–V) characteristics. We found that either annealing at 750 °C under N2, or removal of ∼500–600 A of the surface essentially restored the initial I–V characteristics. There was no measurable improvement in the plasma-exposed diode behavior with (NH4)2S treatments.

Breakdown voltage and reverse recovery characteristics of free-standing GaN Schottky rectifiers

Schottky rectifiers with implanted p/sup +/ guard ring edge termination fabricated on free-standing GaN substrates show reverse breakdown voltages up to 160 V in vertical geometry devices. The specific on-state resistance was in the range 1.7-3.0 /spl Omega//spl middot/cm/sup 2/, while the turn-on voltage was /spl sim/1.8 V. The switching performance was analyzed using the reverse recovery current transient waveform, producing an approximate high-injection, level hole lifetime of /spl sim/15 ns. The bulk GaN rectifiers show significant improvement in forward current density and on-state resistance over previous heteroepitaxial devices.

Growth and Fabrication of GaN/AlGaN Heterojunction Bipolar Transistor

A GaN/AlGaN heterojunction bipolar transistor structure with Mg doping in the base and Si Doping in the emitter and collector regions was grown by Metal Organic Chemical Vapor Deposition in c-axis Al(2)O(3). Secondary Ion Mass Spectrometry measurements showed no increase in the O concentration (2-3x10(18) cm(-3)) in the AlGaN emitter and fairly low levels of C (~4-5x10(17) cm (-3)) throughout the structure. Due to the non-ohmic behavior of the base contact at room temperature, the current gain of large area (~90 um diameter) devices was <3. Increasing the device operating temperature led to higher ionization fractions of the mg acceptors in the base, and current gains of ~10 were obtained at 300 degree C.

Lateral AlxGa1−xN power rectifiers with 9.7 kV reverse breakdown voltage

AlxGa1−xN (x=0–0.25) Schottky rectifiers were fabricated in a lateral geometry employing p+-implanted guard rings and rectifying contact overlap onto an SiO2 passivation layer. The reverse breakdown voltage (VB) increased with the spacing between Schottky and ohmic metal contacts, reaching 9700 V for Al0.25Ga0.75N and 6350 V for GaN, respectively, for 100 μm gap spacing. Assuming lateral depletion, these values correspond to breakdown field strengths of ⩽9.67×105 V cm−1, which is roughly a factor of 20 lower than the theoretical maximum in bulk GaN. The figure of merit (VB)2/RON, where RON is the on-state resistance, was in the range 94–268 MW cm−2 for all the devices.

Vertical and lateral GaN rectifiers on free-standing GaN substrates

Edge-terminated Schottky rectifiers fabricated on quasibulk GaN substrates showed a strong dependence of reverse breakdown voltage VB on contact dimension and on rectifier geometry (lateral versus vertical). For small diameter (75 μm) Schottky contacts, VB measured in the vertical geometry was ∼700 V, with an on-state resistance (RON) of 3 mΩ cm2, producing a figure-of-merit VB2/RON of 162.8 MW cm−2. Measured in the lateral geometry, these same rectifiers had VB of ∼250 V, RON of 1.7 mΩ cm2 and figure-of-merit 36.5 MW cm−2. The forward turn-on voltage (VF) was ∼1.8 V (defined at a current density of 100 A cm−2), producing VB/VF ratios of 139–389. In very large diameter (∼5 mm) rectifiers, VB dropped to ∼6 V, but forward currents up to 500 mA were obtained in dc measurements.

Effects of interfacial oxides on Schottky barrier contacts to n- and p-type GaN

Schottky contacts were formed on n- and p-type GaN after either a conventional surface cleaning step in solvents, HCl and HF or with an additional treatment in (NH4)2S to prevent reformation of the native oxide. Reductions in barrier height were observed with the latter treatment, but there was little change in diode ideality factor. A simple model suggests that an interfacial insulating oxide of thickness 1–2 nm was present after conventional cleaning. This oxide has a strong influence on the contact characteristics on both n- and p-type GaN and appears to be responsible for some of the wide spread in contact properties reported in the literature.

High voltage GaN Schottky rectifiers

Mesa and planar GaN Schottky diode rectifiers with reverse breakdown voltages (V/sub RB/) up to 550 and >2000 V, respectively, have been fabricated. The on-state resistance, R/sub ON/, was 6 m/spl Omega//spl middot/cm/sup 2/ and 0.8 /spl Omega/ cm/sup 2/, respectively, producing figure-of-merit values for (V/sub RB/)/sup 2//R/sub ON/ in the range 5-48 MW/spl middot/cm/sup -2/. At low biases the reverse leakage current was proportional to the size of the rectifying contact perimeter, while at high biases the current was proportional to the area of this contact. These results suggest that at low reverse biases, the leakage is dominated by the surface component, while at higher biases the bulk component dominates. On-state voltages were 3.5 V for the 550 V diodes and /spl ges/15 for the 2 kV diodes. Reverse recovery times were <0.2 /spl mu/s for devices switched from a forward current density of /spl sim/500 A/spl middot/cm/sup -2/ to a reverse bias of 100 V.

GaN electronics for high power, high temperature applications

A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors and GaN metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.